MADISON, Wis. – A challenge to what 12 Democratic voters claim is “one of the worst partisan gerrymanders in American history” is headed to trial in Wisconsin next month.

The voters sued the individual members of Wisconsin’s Government Accountability Board in 2015, claiming that Republican lawmakers secretly crafted and hurriedly passed a redistricting plan that would give them overwhelming – and unfair – control of the state legislature.

The Government Accountability Board oversees election activity in the state. However, the panel is in the process of being dismantled as the result of reforms signed into law by Gov. Scott Walker in December. In June it will be replaced with new elections and ethics commissions.

On Dec. 17, 2015, a three-judge district court panel denied defendants’ motion to dismiss the case, concluding the plaintiffs’ allegations were sufficient to state a plausible claim.

The defendants then filed a motion for summary judgment, which the three-judge panel denied (pdf) on Thursday.

“Defendants raise many important points in their summary judgment submissions,” says the opinion signed by U.S. District Judges Barbara Crabb and William Greisbach, and Circuit Judge Kenneth Ripple. “It may be that one or more of these objections carries the day in the end.”

“However, we believe that deciding the case now as a matter of law would be premature because there are factual disputes regarding the validity of plaintiffs’ proposed measurement for determining the existence of a constitutional violation.”

The judges also rejected the plaintiffs’ request to exclude the opinions of one of the defendants’ expert witnesses.

The joint rulings set the stage for a trial now scheduled to begin on Tuesday, May 24. In placing the trial on the court calendar, the judges said the parties should be prepared for it to last just four days.

Federal law requires that a three-judge panel to hear redistricting cases, and the tight timeframe was mandates to accommodate scheduling conflicts, the judges said.

In addition to claiming Republicans colluded to gerrymander the state’s election district map, the plaintiffs say they were harmed by the result – legislative elections in 2012 and 2014 in which the GOP gained a significant number of seats in the Wisconsin Assembly.

Not only is the plan unconstitutional, but its efficiency gap – the number of so-called “wasted votes” divided by total votes – is unnecessary, the voters claim.

Wasted votes are those cast beyond what’s needed for a party to win as well as those cast for the losing candidate, according to the lawsuit.

The Democrats have proposed an alternate plan – one they say reduces the “efficiency gap” created by a high number of wasted votes to a neutral level.

“The demonstration plan proves that the current plan’s extreme pro-Republican tilt cannot be blamed on either an effort to comply with legitimate redistricting criteria or Wisconsin’s underlying political geography,” the complaint says. “Both of those factors were perfectly compatible with a neutral map.”

A judge in a prior lawsuit called claims the current plan was not influenced by partisan politics “almost laughable,” according to the complaint.

The defense argues not that the voters cannot meet the standard of proving their claims, but that this “efficiency gap” is a flawed standard by which to measure discriminatory effect, according to the three-judge panel’s opinion.

They argue that Democrats are naturally packed into smaller geographical spaces, which results in a smaller share of legislative seats than votes, the opinion states.

All of these disagreements will now be aired at trial.

Any requests for schedule changes must be submitted by April 18.

The lead attorney for the plaintiffs, Nicholas Stephanopoulos of the University of Chicago Law School, did not immediately respond to a voicemail.

Johnny Koremenos, a special assistant to the attorney general at the Wisconsin Department of Justice, said the court’s rejection of the state’s motion is not a loss, but simply another step in the ultimately resolution of the case.

“Once the court hears all the facts and the legal arguments, it will then issue its final decision on the merits of the lawsuit. Therefore, we can’t challenge or appeal the court’s decision from yesterday,” he said in an email to Courthouse News.